NCPI - Engineering Manual, Chapter 4

This section deals with the examination and evaluation of all those forces which affect or influence the structural stability of a rigid conduit.

Methods are outlined by which trench loads may be considered and analyzed for the purpose of accomplishing required structural support.

Importance of Predetermining Loads Accurately

There is a tendency to think of sewer pipe from the hydraulic standpoint only and to neglect the importance of pipe as a structural element. It must, above all else, maintain structural stability.

Nearly all building codes impose legal standards upon designers to insure against the failure of building structures. Standard practice in highway work and railroad work also provides for predetermined structural safety.

There are accepted methods for insuring against the overloading or collapse of subsurface pipes used to convey sewage. Unfortunately, however, there are not legal restrictions to make these methods mandatory. As a result, there is too much "rule of thumb" loading design. Failure to make adequate comparisons between pipe strength and loads to be resisted is far too common in sewer work.

Designers, who would never consider determining a cross section for a beam without comparing external and internal moments, will frequently permit specifications of a sewer without allowing for such variable factors as the type of pipe bedding, type of backfill material, different trench widths and depths, nature of the foundation, or expected live loads. This apparent carelessness is caused by the habit of thinking of trench loads in terms of trench depth only. Actually, the neglected factors are often much more important, as far as safety is concerned, than is depth of trench.

Loads Can be Accurately Determined

Just as the safety of ordinary structural members involves the application of "mechanics" to cases of assumed live loadings, the safety in underground pipe work involves application of "soil mechanics" for determining the load on the pipe. The amount of load to be supported by the pipe can be computed and the result will be safe and accurate in the same sense that predetermination of beam strength is safe and accurate.

To provide engineers with a convenient method of predetermining loads and strength requirements for clay pipe, complete reference tables are included in Chapter 5. These tables show the estimated load according to pipe size, trench depth and width and type of backfill. Other references (pages 44-55) provide data for determining the effect of the type of bedding or support given the pipe.

The following is a presentation of the Marston theory upon which the Trench Load Tables are based.

Trench Load Equation

To determine a reliable equation for computing the relationship between various kinds of loads and the required test strength of pipe, a series of studies have been made at the Engineering Experiment Station of Iowa State College. The result is the Marston Equation named for its originator, Anson Marston, who was President of the American Society of Civil Engineers and Dean Emeritus of the College. It is a widely recognized, conservative equation for computing trench loads on pipe.

An understanding of the Marston Equation, and the factors involved, is helpful when using the trench load tables.

Essentially, any structure installed below the surface of the earth supports the weight of all the materials above it, depending upon certain characteristics of the fill. These characteristics, (principally internal soil friction) tend to increase or diminish the gross weight of the material above the pipe structure.

This is true for both trench and embankment loads. Considering a structure of circular cross-section such as a sewer pipe, the backfill material directly above the pipe is that material which lies between vertical planes tangent to the outside of the pipe barrel. The net load on the pipe exclusive of live load, is the actual weight of such backfill material plus or minus an amount which depends on whether internal soil friction assists in the support of the mass of backfill over the pipe or not.

The drawing above demonstrates the cross-section of a typical sewer trench showing the location of planes tangent to the sides of the pipe. These are called the primary planes. When the backfill in a trench is compacted uniformly, uniform settlement (further compaction) can be expected with the passage of time. The depth of the backfill between the primary tangent planes will be reduced through such settlement by a fairly definite amount, depending upon the nature and compaction of the original backfill.

The backfill between the trench walls and the primary planes on either side of the pipe will also settle in time.

Frictional Forces in the Backfill

Since the depth of the backfill between the pipe and the trench sidewalls is greater than the depth of the backfill directly over the pipe, it will settle or compact more than the material directly over the pipe. This movement will be restricted by friction between the back-fill particles on each side of the primary tangent planes. The increased settlement of the backfill on both sides of the pipe tends to pull down the mass of backfill over the pipe, and thereby transmit an additional load to the pipe.

Secondary vertical planes are assumed anywhere between the primary planes and the walls of the trench as shown in the drawing. As mentioned previously, the backfill between the primary and secondary planes is prevented from settling to a maximum amount by the action of a frictional force along the primary vertical planes. This increases the load supported by the pipe.

The remainder of the backfill which lies between the secondary planes and the trench walls is supported in part by friction along the trench walls. This reduces the load on the pipe.

The Effect of Trench Width

It will be seen that, as the secondary plane is moved away from the pipe, the differential settlement on opposite sides of the plane will become less. It is therefore possible to locate a definite position where the differential settlement on opposite sides of the secondary plane is so small that no frictional forces are transmitted across it. When this location is within the cross section of the trench, the weight of backfill between the secondary plane and trench wall can add nothing to the load on the pipe. In other words, the trench width may be increased from this point on without adding to the weight on the pipe.

The minimum distance which meets the above qualifications is called the transition width of the trench. It is the trench width at which further widening will have no effect on the load on the pipe.

When the actual width is less than the transition width, friction in the plane of the trench wall tends to support part of the load and to lessen the load on the pipe. This phenomenon is graphically illustrated by the curve marked surface curve after settlement in the previously referenced drawing. Wherever this curve deflects downward from its origin directly over the center of the pipe, internal friction in the backfill transmits weight to the pipe. Where the curve deflects upward (as alongside the trench wall) backfill weight is transmitted to the side wall of the trench.

Marston Equation

The Marston Equation applies the preceding reasoning to the calculation of loads on pipes. Actual tests have been made with many kinds of soil to determine their weights and frictional characteristics and to determine the relative settlement of each. These measurable quantities have been combined into a single expression to produce for each case a computation of the total weight supported by the pipe.

By substitution of available data in the Marston equation, a direct result is obtained for the load on the pipe in terms of pounds per linear foot. The computation of loads is simplified by the use of this equation and the Computation Diagram on page 37, which represents the plotted solution of the "Load Calculation Coefficient" equation:

The Computation Diagram is based on various types of soil conditions, and may be used to obtain the values of the load calculation coefficient "C_d."

The Trench Load Tables in Chapter 5 have been compiled using the Marston equation described above. The soil weights are based upon an arbitrary value of 100 lbs./cu.ft. The values used in the calculations are as follows:

When the actual soil weight is known to vary from the estimated value, the tabulated loads may be adjusted up or down by direct ratio.

Embankment Loads

Although these Trench Load Tables show loads on pipe in trenches, they are equally applicable for pipe installed under embankment conditions. As the width of the trench increases, other factors remaining constant, the load on the pipe increases until it reaches a limiting value equal to the embankment load on the pipe. This limiting value is called the "transition width". The transition widths shown in the Trench Load Tables have been calculated using the equation for positive projecting conduits in wide trenches.

Superimposed Loads

Concentrated and distributed superimposed loads should be considered in the structural design of sewers, especially where the depth of earth cover is less than 8 ft. Where these loads are anticipated, they are added to the predetermined trench load. Superimposed loads are calculated by use of Holl's and Newmark's modifications to Boussinesq's equation (page 40).

Concentrated Loads 4nc007.gif (5757 bytes)

Holl's integration of Boussinesq's solution leads to the following equation for determining loads due to superimposed concentrated load, such as a truck wheel load (Diagram 1, page 39):

in which W_sc is the load on the conduit in lb/ft of length; P is the concentrated load in lb; F is the impact factor; C_s is the load coefficient, a function of B_c/(2H) and L/(2H); H is the height of fill from the top of conduit to ground surface in ft; B_c is the width of conduit in ft; and L is the effective length of conduit in ft. (For values of F and C_s, see pages 40 and 41).

An effective length, L, equal to 3 ft. for pipe greater than 3 ft. long, and the actual length for pipe shorter than 3 ft. is recommended.

Unless other data are available, it is safe to estimate that truck wheel loads are the greatest loads to be supported. H-20 wheel loadings are standard for highway and bridge design and are equally applicable for estimating loads on sewers.

H-20 refers to loading resulting from the passage of trucks having a gross weight of 20 tons, 80% of which is on the rear axle, with axle spacing of 14 ft., center to center, and a wheel gauge of 6 ft., each rear wheel carrying one half this load or 8 tons each without impact.

Example: Determine the load on a 15 inch, 5 ft. length of pipe under 5 ft. of cover caused by a concentrated H-20 wheel load.

P = 16,000 lb; F = 1.5 (Highway); L = 3.0 ft. (for pipe greater than 3 ft. long); d= pipe diameter = 15 inch; t = wall thickness = 1.5 inches. Then B_c = 15 + 3 = 18 inches = 1.5 ft.; H = 5.0 ft.; B_c/(2H) = 1.5/10 = 0.15; L/(2H) = 3/10 = 0.30; and by interpolation from the table shown on page 40, C_s is 0.078. Substituting in the equation:

If the concentrated load is not centered vertically over the pipe, but is displaced laterally and longitudinally, the load on the pipe can be computed by adding the effect of the concentrated load. Dividing the tabular values of C_sby 4 will give the effect for this condition.

An alternative method of determining concentrated live or superimposed loads on a buried conduit, is to use the Percentages of Wheel Loads shown in the Table on page 40. These percentages have been determined directly from data contained in "Theory of External Loads on Closed Conduits," Bulletin 96, published by the Engineering Experiment Station at Iowa State College. Note that an allowance for impact must be added to the percentage figures shown in the table. The table does not apply to distributed superimposed loads.

Example: Referring to the previous example problem, P = 16,000 lb; F = 1.5 (Highway); and from the Percentage of Wheel Load Table, page 40, percentage of load for 15-inch pipe with 3 ft. depth of cover = 6.4%.

Distributed Loads

For determining loads on pipe due to superimposed loads distributed over a surface area (Diagram 2) the following equation was developed:

in which W_sd is the load on the conduit in lb/ft of length; p is the intensity of distributed load in psf; F is the impact factor; B_c is the width of the conduit in ft; C_s is the load coefficient, a function of D/(2H) and M/(2H); H is the height from the top of the conduit to the ground surface in ft; and D and M are the width and length, respectively, of the area over which the distributed load acts, in ft. (For values of C_s and F, see pages 40 and 41.)

If the area of the distributed superimposed load is not centered vertically over the pipe, but is displaced laterally and longitudinally, the load on the pipe can be computed by adding algebraically the effect of various rectangles of loaded area. It is more convenient to work in terms of load under one corner of a rectangular loaded area rather than at the center. Dividing the tabular values of C_s by 4 will give the effect for this condition.

Dynamic Loads

Impact factors must be considered to account for the influence of dynamic loading due to traffic. The following table shows suggested values.

Trench Width

To properly approach the analysis of loads imposed on the pipe, it is necessary to decide, for each size of pipe, what the minimum practicable design trench width at the top of the pipe is to be and still permit good workmanship. The design trench width, the depth of fill over the pipe, and the soil characteristics of the fill, will produce the load which must be supported by the pipe and its bedding. This load is readily available from either the Trench Load Tables or the NCPI trench load computer program when the above factors are known.

Using Trench Load Tables

The correct use of the Trench Load Tables which are given in Chapter 5 is demonstrated by the following hypothetical case where a designer wants to know the trench load imposed by the following conditions:

A 12-inch sewer is to be installed in an area of sand and gravel with an average weight of 100 lb./cu. ft. The top of the pipe is 8 ft. below ground surface and the trench width is 30 inches.

Pipe size -	12 in.
Backfill -	Sand and Gravel
Trench width -	30 in.
Backfill weight -	100 lb./cu. ft.
Cover depth -	8 ft.

From the Trench Load Tables, the load is 1240 lb./lin. ft.

Typical Load Computation for Highway Work

Suppose that plans call for the installation of a 15-inch sewer line with 5 ft. of cover in a 3 ft. wide trench of wet clay weighing 130 lb/cu. ft. and that construction equipment wheel loads of 16,000 lbs. each will pass over the backfilled trench before the pavement is placed. (This is the maximum loading condition) What is the total load on the pipe? To determine the trench load use the Trench Load Table on page 69.

Pipe size -	15 in.
Backfill -	Wet Clay
Trench width -	36 in.
Backfill load (1170 lb./lin. ft. x 130/100) =	1520 lb./lin. ft.

(The live load has been calculated. See example on page 39.)

Live load =	624 lb./lin. ft.
Total trench load =	2144 lb./lin. ft.

Note that the concentrated and distributed live load equations shown on pages 38 and 41 include an allowance for impact (F), and that values of F are listed on page 41.

In computing the load to be supported by the pipe line illustrated above, live load must be added to the backfill load.

SUMMARY

Part I has dealt with the examination and evaluation of those forces which affect or influence the structural stability of a rigid conduit, underground. Having completed the analysis of determining LOADS on the pipe, the method of designing the pipeline to SUPPORT these loads will be developed in Part II - Structural Design.

RIGID CONDUITS, UNDERGROUND

Part II - Structural Design

This section deals with the structural design as outlined by the analysis of the trench loads previously developed in Part I, "Structural Analysis". Structural support is achieved by selecting and providing proper trench and bedding conditions. The following text describes the methods by which the trench loads must be supported.

It is of fundamental importance to recognize the variable supporting strengths of pipe in the trench, including a design factor of safety, under various bedding and field construction conditions.

The several factors influencing the structural stability of the proposed installation must first be considered. These factors include:

When these factors have been taken into consideration, the supporting strength of Vitrified Clay Pipe can then be calculated.

"Design Load" Versus "Actual Load"

The design load is the actual load adjusted by a factor of safety. It is determined by dividing the field supporting strength by the total trench load.

It should be clearly recognized that all loads considered in Part I, have been the actual loads imposed upon a conduit in a given installation. From this point on all loads must be translated into design loads because it is only through this translation that the factor of safety is incorporated in the design.

An engineer determines the factor of safety based upon his knowledge of local soil conditions, construction practices, future development of the area and any unusual variations of land use. Solutions to sample problems are shown on pages 55 and 56.

The Effect of Trench Width

The trench width at the top of the pipe is one of the most important factors involved not only in design but throughout construction.

As shown in the equation W_c= C_dwB_d², the load on the pipe increases in relation to the square of the trench width. Therefore, even a relatively small increase in width results in a large increase in load.

For example, an 8 inch pipe laid in a sand and gravel soil weighing 100 pounds per cubic foot at a cover depth of 14 feet and 24 inch trench width will have a load imposed of 1090 lbs./lin.ft. If the trench width is increased only 6 inches to 30 inches, the load imposed will increase to 1600 lbs./lin.ft., or almost 50 percent.

The design trench width at the top of pipe equals the sum of the outside diameter of the pipe, the minimum working space on each side of the pipe, and the thickness of sheeting if removed or the trench box wall on each side of the trench.

Controls during the course of construction which will preserve the design trench width are vital to the success of the underground structure.

The Effect of Moving the Trench Box or Removing the Sheeting

When a trench box is moved or sheeting is removed from a trench after bedding has been placed, a space is created at the sides of the trench. Bedding material slips into this space and robs the pipe of part of its lateral and vertical support.

Good engineering practice recommends that timber sheeting be cut off at the top of the pipe and the upper portion removed without harming the support conditions. Thin steel sheeting may be carefully withdrawn.

The Effect of Sloping Trench Walls

Since the load on the pipe increases with the square of the width of the trench at the top of the pipe, it follows that trenches should be as narrow as possible.

All available evidence shows that the width or shape of the trench above the level of the top of the pipe does not increase the load on the pipe. The trench walls above that level may be sloped outward without adding to the load on the pipe.

Supporting Strength of Vitrified Clay Pipe

1. Physical Properties of Vitrified Clay Pipe

Tests to determine the unit strength of vitrified clay pipe are consistent throughout the country. The tests are established by the American Society for Testing and Materials, as set forth in ASTM C 301, Standard Methods of Testing Vitrified Clay Pipe.

Vitrified clay pipe are tested and certified at the place of manufacture by the manufacturer to determine the bearing strength in terms of pounds per linear foot. This may be observed by the engineer in charge of construction, or his representative. Vitrified clay pipe may also be tested by independent testing laboratories when designated by the engineer.

Test procedures to determine the acid resistant qualities and absorption or hydrostatic resistance of vitrified clay pipe are also outlined in ASTM C 301.

2. Proper Bedding to Develop Design Supporting Strength

To obtain the installed supporting strength in accordance with the class of bedding used, the pipe barrel must be uniformly supported by direct contact with firm bedding.

Firm bedding means the pipe barrel must rest on undisturbed native or imported material. The native material in the trench bottom must be capable of excavation to a uniform undisturbed flat bottom in the case of Class D. If the trench is over-excavated, the trench bottom should be brought back to grade with the required bedding material.

NOTE: Shovel-slicing the bedding material in the haunch areas is of great benefit to the installed pipe. The ease with which shovel-slicing is accomplished suggests that it should be considered as standard procedure for all classes of bedding, especially for Class B. It takes little time, assures that the pipe will remain true to grade, eliminates voids beneath the pipe and in the haunch areas, consolidates the bedding where it is needed the most, and adds little or nothing to the cost of the installation. To be the most effective, shovel-slicing should be done before the bedding is brought up to the springline, preferably when it is no higher than the quarter point of the pipe.

Bell or coupling holes should be carefully excavated so that no part of the load is supported by the bells or couplings. Properly constructed bell or coupling holes are necessary to provide uniform support. Best results are obtained when the bell or coupling holes are loosely backfilled. Consolidation of material around and under the bell and couplings during bedding and backfilling should be avoided.

Provide uniform and continuous support of pipe barrel between bell or coupling holes for all classes of bedding.

The field supporting strength of the pipe is substantially reduced when the pipe is improperly bedded. The engineer should insure that the class of bedding specified is actually provided during construction. The absolute need of complete control during construction is clearly demonstrated by significant losses in the field supporting strength of the pipe as a result of improper bedding.

Provide uniform and continuous support of pipe barrel between bell or coupling holes for all classes of bedding.

3. Bedding Materials

Imported Bedding
Applied research and subsequent general acceptance in the field calls attention to the advantages of interlocking bedding materials, such as crushed stone, with at least one fractured face. It should range in size from 1 in. to 1/4 in., depending on pipe diameter. Standard size numbers for bedding materials shown below are in accordance with ASTM D 448 Standard Classification for Sizes of Aggregate for Road and Bridge Construction (page 48).

Native Bedding
Many native materials taken from the trench will provide suitable support for clay pipe and may be the most cost efficient method of installation. Care must be exercised to remove large stones which could cause point loading. The local material must have previously demonstrated satisfactory performance by common practice and be used only when the required load factor design will not be compromised.

General Guidelines

The guidelines listed above are by no means complete or applicable to every situation and are offered as a basis from which judgment and practical application may be made.

4. Load Factors

The load which a pipe can support varies according to the class of bedding selected.

Trench details shown on page 49 depict the recommended classes of bedding and cradling. Load factors have been determined for each. The load factor is the ratio of the supporting strength of the pipe in the trench to its three-edge bearing test strength. It does not include a factor of safety. The three-edge bearing strength has been established as a base and is considered equivalent to a load factor of 1.0.

Field Supporting Strength

The load factor is used to compute the supporting strength of vitrified clay pipe with any designated type of bedding or cradling. Thus, the three-edge bearing strength of vitrified clay pipe is multiplied by the appropriate load factor to obtain the installed field supporting strength of the pipe produced by the specific type of bedding or foundation selected. (Field supporting strength = load factor x pipe three-edge bearing strength.) Therefore, it is possible to provide the necessary field supporting strength to exceed the calculated trench loads. (Field supporting strengths of extra strength clay pipe are shown in the table on page 49.)

Supporting Strength in Trench Conditions

The pipe may be laid on a flat or unshaped trench bottom of suitable undisturbed native material or, in the case of overexcavating, on a restored flat bedding base. In either case the bottom of the entire pipe barrel shall have a continuous and uniform bearing support. The initial backfill shall be of select material (NOTE 2 page 55).

The pipe shall be bedded in granular material carefully placed (NOTE 1 page 55) on a firm trench bottom with a minimum thickness beneath the pipe of 4 inches or one-eighth of the outside diameter of the pipe, whichever is greater, and sliced under the haunches of the pipe with a shovel or other suitable tool to a height of one-sixth of the outside diameter of the pipe. Crushed stone, gravel or other locally available non-cohesive materials may be used. The initial backfill shall be of select material (NOTE 2).

The pipe shall be bedded in crushed stone or other suitable material which is non-consolidating and not subject to migration. The bedding shall be carefully placed (NOTE 1 page 55) on a firm trench bottom with a minimum thickness beneath the pipe of one-eighth the outside pipe diameter, but not less than 4 inches and sliced under the haunches of the pipe with a shovel or other suitable tool to a height of one-half the outside pipe diameter, or to the horizontal centerline. Shovel-slicing the bedding material under the haunches of the pipe is essential if the total load factor is to be realized. The initial back-fill shall be of select material (NOTE 2).

There are specific sites where a load factor of 2.2 may be desirable. The pipe shall be bedded and encased in crushed stone or other suitable material which is non-consolidating and not subject to migration. The bedding shall be placed on a firm trench bottom with a minimum thickness beneath the pipe of one-eighth the outside pipe diameter, but not less than 4 inches and sliced under the haunches of the pipe with a shovel or other suitable tool. Shovel-slicing the bedding material under the haunches of the pipe is essential if the total load factor is to be realized. The encasement material shall extend laterally to the specified trench width and upward to a horizontal plane at the top of the pipe barrel following removal of any trench sheeting or boxes. The initial backfill shall be of select material (NOTE 2).

The pipe shall be bedded on crushed stone or other suitable material. The bedding shall have a minimum thickness beneath the pipe of 4 inches or one eighth of the outside diameter of the pipe, whichever is greater. CDF shall be carefully discharged to the top of the pipe and allowed to flow approximately equally to both sides to prevent misalignment. The fill can be made in a single pour to the top of the pipe or it can be made in two or more lifts if desired. It is recommended that the CDF material be continuously agitated in the delivery truck due to rapid segregation of materials. This is particularly important just before pouring. The material shall have a 28 day compressive strength of 100-300 psi. A suggested mix for one cubic yard is as follows: 100 lbs. cement, 250 lbs. fly ash, 2700 lbs. sand and approximately 60 gallons of water. The material should flow with near water-like consistency. Other proportions may be approved if satisfactory local experience is available. The initial backfill may be placed when the CDF is capable of supporting the backfill material without intermixing. This will normally be about 20 to 30 minutes or when the penetrometer readings are greater than 62.5 lbs./sq. ft. Further evaluation may be necessary where native soils are expansive.

Three types of Class A bedding are illustrated giving the designer a wide selection of load factors. It is therefore possible to select an adequate Class A bedding to meet most design conditions. The angular material shall be crushed stone or other suitable material which is non-consolidating and not subject to migration. The concrete shall be 3000 psi or greater strength.

NOTE 1: Carefully placed material shall mean material that has been spaded or shovel-sliced so that the material fills and supports the haunch area and encases the pipe to the limits shown in the trench diagrams.

NOTE 2: The initial backfill shall be placed from the bedding to a level of 12 inches over the top of the pipe and shall consist of select, finely divided material free of debris, organic material and large rock and stones.

When making trench load calculations for clay pipe, a safety factor within the range of 1.0 to 1.5 is desirable. This can be accomplished by using the different bedding classes for VCP (ASTM C 12).

Bedding design must be both structural-ly sufficient and cost effective. The fol-lowing computations illustrate these procedures.

Sample Problems

Assume a 24-inch clay pipe line is to be installed in an area of dry clay which has an average weight of 120 pounds per cubic foot. The depth of cover over the top of the pipe is 18 feet and the trench width at the top of the pipe is 48 inches. Determine a structurally sound and economic bedding design.

The trench load is determined by using the Trench Load Tables, the NCPI trench load design computer program or other means.

With the trench load on the pipe determined, the next step is to calculate the field supporting strength and the safety factor.

ASTM 3-edge bearing strength, 24-inch Extra Strength pipe = 4400 lbs/lin. ft. (This bearing strength applies to the following three examples.)

Conclusions

5. Use of Concrete to Develop Added Support

The following methods of using concrete to increase the field supporting strength of clay pipe are:

The pipe is placed to line and grade on supports under the barrel. Concrete is poured from the bottom of the trench up to a height of one-fourth of the outside diameter of the pipe. The load factor varies with the cross-sectional area of reinforcing steel in the transverse direction.

This method of increasing the supporting strength of pipe requires the pipe to be bedded in angular material to the springline. Concrete is placed on the bedding and extends over the top of the pipe. The load factor varies with the cross-sectional area of steel reinforcing in the transverse direction. Solid support for the legs of the arch are critical to satisfactory performance.

This method of encasement can be used when other systems do not provide the required strength. It is also used to span areas of unstable soils where the pipe-concrete composite system must be designed as a beam. Although this type of installation carries a load factor designation, the plan details should be reviewed by an engineer experienced in structural concrete design.

Principles of Concrete Design

The use of concrete bedding permits the pipe to support substantially higher backfill loads than are normally possible. However, immediate backfilling of the trench may prevent the full development of the design strength of the pipeconcrete system. When this occurs the pipe is likely to be placed in the position of supporting the entire trench load without any assistance from the concrete.

Consideration of the following items may help avoid the problems associated with the use of concrete.

The trench must not be backfilled before the concrete has gained sufficient strength to support the backfill load without cracking. A minimum of two days is recommended. Although it may be impractical to delay backfilling longer than this, it is obvious that the strength of the pipe-concrete system is still in a structurally critical stage.

When the trench backfill is allowed to consolidate through natural means, the maximum load on the pipe will be delayed. However, paving requirements and other considerations such as traffic flow may preclude the possibility of extended delay. In those instances the engineer and or contractor must evaluate the possible risks involved.

Early strength increase is normally accomplished by increasing the cement content, adding accelerators or by the use of Type III high early strength Portland cement.

The addition of fly ash or other pozzolanic material may retard early strength development and should not be used.

A common method of increasing the strength of the concrete, although not necessarily early strength, is through the use of steel reinforcement. The strength increase is generally in proportion to the cross-sectional area of steel to the concrete above or below the pipe. The steel should be placed in the transverse direction to the pipe.

In concrete cradle construction, the percentage of reinforcement, p, is the ratio of the area of transverse steel reinforcement to the area of concrete at the pipe invert above the center line of the reinforcement.

In arch construction, the percentage of reinforcement, p, is the ratio of the transverse reinforcement to the area of concrete above the top of the pipe and below the centerline of the reinforcement. Reference ASCE #60, WPCF MOP FD-5 1982, Pg. 204.

Welded steel wire fabric is recommended for use in load factor design because of its uniformity and relative ease of installation.

Consideration should be given to the possibility of pipe flotation associated with the use of concrete.

	k_u' (Dimensionless)
Sand & Gravel	0.165
Saturated Topsoil	0.150
Dry Clay	0.130
Wet Clay	0.110

IMPACT FACTORS (F)
Traffic	Impact Factor
Highway	1.50
Railway	1.75
Runways/Airfield	1.00
Taxiways, aprons, hardstands	1.50

Nominal Pipe Size	ASTM D 448 Size

Less than 15 in.	67, 7 or 8
15 in. to 30 in.	6 or 67
Greater than 30 in.	57, 6 or 67

Pipe size -	24 in.
Depth of cover -	18 ft.
Backfill -	dry clay @ 120 lbs/cu. ft.
Trench width -	48 in.
(From Trench Load Table)
	4240 x 120/100
Total Trench Load =	5090 lbs/lin. ft.

Clay Pipe Engineering Manual

Chapter 4, Rigid Conduits, Underground

Part I - Structural Analysis

Importance of Predetermining Loads Accurately

Loads Can be Accurately Determined

Trench Load Equation

Frictional Forces in the Backfill

The Effect of Trench Width

Marston Equation

Embankment Loads

Superimposed Loads

Concentrated Loads

Distributed Loads

Dynamic Loads

Trench Width

Using Trench Load Tables

Typical Load Computation for Highway Work

SUMMARY

RIGID CONDUITS, UNDERGROUND

Part II - Structural Design

"Design Load" Versus "Actual Load"

The Effect of Trench Width

The Effect of Moving the Trench Box or Removing the Sheeting

The Effect of Sloping Trench Walls

Supporting Strength of Vitrified Clay Pipe

1. Physical Properties of Vitrified Clay Pipe

2. Proper Bedding to Develop Design Supporting Strength

3. Bedding Materials

General Guidelines

4. Load Factors

Field Supporting Strength

Supporting Strength in Trench Conditions

Sample Problems

Conclusions

5. Use of Concrete to Develop Added Support

Principles of Concrete Design


Clay pipe can be installed in deep trenches with appropriate design and proper installation.	Vitrification produces a strong and inert body composition which enables a properly installed clay pipe system to permanently support the trench loads.